The present invention relates to a multilayered air-fuel ratio sensor used for controlling an air-fuel ratio of a gas mixture supplied to a combustion chamber of an internal combustion engine.
To suppress energy loss (i.e., fuel loss) and prevent serious air pollution, using an air-fuel ratio sensor is inevitably required for present passenger vehicles.
FIGS. 13 and 14 show a conventional multilayered air-fuel ratio sensor disclosed in Japanese Patent No. 2-62955 corresponding to U.S. Pat. No. 5,288,389.
As shown in FIG. 13, a multilayered air-fuel ratio sensor 9 comprises multiple layers consisting of a solid electrolytic substrate layer 91, an insulating spacer 92, a solid electrolytic substrate layer 93, and a shielding plate 94.
As shown in FIG. 14, the multilayered air-fuel ratio sensor 9 comprises a pump cell 919 and a sensor cell 939. A sample gas chamber 920 is interposed between the pump cells 919 and 939. A reference gas chamber 940 is provided between the sensor cell 939 and the shielding plate 94. Each of the solid electrolytic substrate layers 91 and 93 and the shielding plate 94 is made of zirconia. The insulating spacer 92 is made of alumina.
The pump cell 919 consists of the solid electrolytic substrate layer 91 and a pair of porous electrodes 911 and 912 provided on opposite surfaces of the solid electrolytic substrate layer 91. The sensor cell 939 consists of the solid electrolytic substrate layer 93 and a pair of electrodes 931 and 932 provided on opposite surfaces of the solid electrolytic substrate layer 91. A sample gas diffusive inlet portion 921 introduces a sample gas to the sample gas chamber 920. A protector layer 900 is provided on an outer surface of the porous electrode 911.
The pump cell 919 maintains the concentration of an oxygen gas residing in the sample gas chamber 920 at a constant value by adjusting an oxygen gas amount introduced to or exhausted from the sample gas chamber 920. The sensor cell 939 detects an air-fuel ratio of the sample gas stored in the sample gas chamber 920.
More specifically, a comparator 950 compares a sensing signal of the sensor cell 939 with a reference voltage. A voltage responsive to an output of the comparator 950 is applied to the pump cell 919. The oxygen gas amount varies in accordance with the applied voltage. Thus, an adjusted oxygen gas is introduced into or exhausted from the sample gas chamber 920. This realizes a feedback control of the concentration of the oxygen gas in the sample gas chamber 920. An obtained current during this feedback control is proportional to an air-fuel ratio of the sample gas. Thus, the air-fuel ratio is detectable from the measured current value.
In general, the air-fuel ratio sensor functions properly only when it has a high temperature exceeding a predetermined active temperature. Hence, to assure an accurate operation, the multilayered air-fuel ratio sensor 9 is equipped with a heater. The heater generates a sufficient amount of heat to maintain the multilayered air-fuel ratio sensor 9 at a higher temperature exceeding its active temperature.
The ULEV law, effective from the year of 2,000 in California State of the Unites States, forces the automotive makers to clear the required levels of tough emission controls. To attain this goal, having an excellent warmup ability is an essential factor to be realized for the above-described multilayered air-fuel ratio sensor.
The planned target levels are significantly high. For example, an air-fuel ratio sensor must operate properly within a short period of 5 seconds immediately after the engine is started up.
In this respect, the above-described conventional multilayered air-fuel ratio sensor 9 has a drawback in that its heater is provided as a separate component. According to this arrangement, the heater must increase its temperature excessively to satisfy the rough regulations. The multilayered air-fuel ratio sensor is subjected to severe thermal shocks. It possibly causes cracks.
As one of practical methods for reducing the thermal shocks, it may be possible to reduce an overall thickness of the multilayered air-fuel ratio sensor. A heat capacity of the multilayered air-fuel ratio sensor decreases in proportion to the reduction of its thickness. However, the mechanical strength of the multilayered air-fuel ratio sensor decreases correspondingly. This is not desirable.
The multilayered air-fuel ratio sensor usually receives various external forces and vibrations, for example, when the multilayered air-fuel ratio sensor is assembled with the heater or when the multilayered air-fuel ratio sensor is installed in an exhaust passage of an internal combustion engine. Accordingly, any multilayered air-fuel ratio sensor suffering from a decreased mechanical strength will be damaged by such external forces and vibrations.
FIG. 15 shows a proposed arrangement for the above conventional multilayered air-fuel ratio sensor 9. A multilayered heater 99 is integrated with the multilayered air-fuel ratio sensor 9 via an insulating substrate layer 990. However, according to this arrangement, the size of the multilayered air-fuel ratio sensor 9 is substantially restricted by the heat ability of the multilayered heater 99. As described above, increasing the heater temperature will cause the problem that the multilayered air-fuel ratio sensor 9 is subjected to severe thermal shocks. If the thickness of the multilayered air-fuel ratio sensor 9 is reduced to solve this problem, the mechanical strength will be fatally deteriorated.